Calculating Flow Rates/Pressure Drops in UFH for Heat Pumps

Calculating flow rates and pressure drops in UFH for heat pumps is important. You need to make sure the figures don’t exceed the limit for your heat pump.

underfloor heating pressure

Having determined the required flow and return water temperatures of your underfloor heating system, you can then move on to calculate the water flow rates and pressure drops for each loop leading from the manifold.

These must be calculated as early as possible during the design process in order to be provided to the heat pump manufacturer tasked with checking the supplied data with the design flow rates of the heat pump.

Hopefully, the flow rates and pressure drops supplied to the manufacturer will sit within the limits of the heat pump. However, if the pressure drops and flow rates exceed the limits of the heat pump, then you will require additional pumps, piping, controls, and wiring. In this situation, the heat pump manufacturer will be able to advise what is required.

Precaution: Check Compatibility:

It is important that that the connection and control detail required by the manufacturer is checked and made sure to co-ordinate with the underfloor heating requirements of your system. Different heat pump manufacturers’ units have different connection requirements.

In cases where this information is not checked at the design stage of the process, a greater risk exists of the underfloor heating system or heat pump not functioning as it should.

Note: Generally, when using small or domestic heat pumps the circulation pump is included within the heat pump. However, commercial heat pumps may not operate in this way.

Calculating the Flow Rate of your Underfloor Heating System:

This calculation is worked out by the same method as for any other water-based heating system.  The water flow rate can be calculated using the following simple formula: M= Q / Cp.∆T


m is the mass flow rate (inkg/s)

Q is the heat output required (in kW)

Cp is the specific heat of water (4.18 kj/kg.K)

T is the temperature difference (K)

For example, the heat output is 55W/m² and temperature drop is 5K. Assuming the room sixe is 20m², the total heat output is 1100W (1.1kW). This calculates a mass flow rate of 0.053kg/s.

Underfloor Heating Pressure Drops:

In certain system designs the pump pressure may require sizing based on the total resistance of the underfloor heating system, taking into account the boiler, distribution piping, manifolds, and the underfloor piping.

However, if the pump serves only the manifold and underfloor loops it should be sized accordingly, and another pump should be provided for the primary circuit.

The water pressure drop depends upon the arrangements of the hydraulics of the system and the type of piping which has been used. Manufacturers and underfloor heating specialists should be able to supply you with details of the frictional properties of their particular underfloor heating pipe.

In cases where the pressure drop within a particular loop is too high due either to the length of the loop of the required flow rate, there are two possible solutions:

  1. Split the single loop into two separate loops;
  2. Increase the pipe diameter: this will reduce the flow velocity and pressure drop, and will also affect the heat output per unit area of floor if the same spacing is used. This is detailed in standard BS EN 1264.

Pressure testing your Underfloor heating system before use

Before the final screed or floor covering is laid, it is vital that a final pressure test is carried out on the newly installed system. To do this, a hydraulic pressure test pump is used.

To carry out the pressure test, follow these steps:

1. Close the flow and return isolating valves;

2. Open each of the black caps and glass flow meters;

3. Connect the hydraulic test pump to the flow manifold header with a hose union. Open fill connector with square vent key;

4. Pump the system to 6 bars for one hour minimum. A small drop in pressure occurs as the pipe expands. Pressure will then stabilise, and the test pump pressure should be checked periodically during the testing time to make sure the pressure remains constant;

5. Keep the system under pressure whilst the screed is poured, or the floor covering is laid. Damage incurred at this point will result in decreased pressure. If this happens, the damage should be repaired and the system re-pressurised before continuing;

6. Pay special attention if installing the system in freezing conditions. Water expands by 10% when frozen, and this can split pipes in a pressurised system.

7. Add a glycol antifreeze mixture to the system to prevent any frozen water clogging the system before start up. Replace this with clean water.

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